Generally, exhaust gas means gas emitted into the air through an exhaust system. Exhaust gas, resulting from air and fuel combusted in a combustion chamber of an engine, includes harmful substances such as carbon monoxide (CO), nitrogen oxide (NOx) and unburned hydrocarbons (HC).
Further, even though a diesel engine generates excellent fuel efficiency and power, unlike a gasoline engine, the exhaust gas from a diesel engine includes a large quantity of nitrogen oxide and particulate matter (PM).
Recently, as regulations for exhaust gases of a vehicle are tightened, as post-processing systems for reducing harmful substances in the exhaust gas, a diesel oxidation catalyst (DOC), a diesel particular matter filter (DPF), selective catalytic reduction (SCR) systems, or the like have been applied.
Among them, the diesel oxidation catalyst (hereinafter, referred to as ‘DOC’) and the diesel particular matter filter (hereinafter, referred to as ‘DPF’) serve to reduce the particulate matter (PM) in the exhaust gas of the diesel engine.
Referring to FIG. 1, an exhaust gas post-processing system includes a DOC 1 configured to combust unburned components in exhaust gas of an exhaust pipe of an engine 4 by a catalytic action, a DPF 2 configured to collect a particulate matter in the exhaust gas, an exhaust valve 5, temperature sensors 6 and 7 configured to each detect temperatures of the exhaust gas of an inlet of the DOC 1 and an inlet of the DPF 2, a differential pressure sensor 8 configured to detect a differential pressure of front and rear sides of the DPF, and the like.
The DPF 2 requires and employs a regeneration process for removing soot, or black dirt, of the particulate matter, which is deposited at a predetermined level or higher and the temperature of the DPF 2 rises to an ignition temperature of the soot, or higher, during the regeneration process to combust and remove the soot.
The DPF regeneration is performed according to a result of detecting the differential pressure by the differential pressure sensor 8, or is performed when a DPF relevant mileage accumulated by a vehicle reaches a predetermined value (that is, when a DPF relevant mileage reaches a DPF regeneration period according to a distance driven by the vehicle).
The soot collected in the DPF is burned off by hydrocarbon injection (HCl) of hydrocarbon (HC) (fuel) into the exhaust pipe of the front side of the DOC 1 at the time of the regeneration of the DPF 2 to increase and/or maintain the temperature of the exhaust gas. In this case, an injector 3 that injects fuel into the exhaust pipe is used.
That is, the temperature of the exhaust gas introduced into the DOC (1) is increased by injecting fuel into the front side of the DOC 1 through the injector 3, the temperature of the exhaust gas is increased by using reaction heat emitted at the time of a generation of NO2 in the DOC 1, and the DPF 2 forcibly combusts the soot by the temperature of the high-temperature exhaust gas.
Meanwhile, a method for increasing a temperature of exhaust gas in multiple stages to secure a stable regeneration temperature at the time of the generation of the DPF has been applied. FIG. 2 illustrates a method for increasing temperature in a multiple stages.
FIG. 2 illustrates a method for securing a regeneration temperature by a two-stage temperature raising process after the regeneration mode entry. A one-stage temperature raising process of increasing initial idle temperature (cold) to a given temperature (for example, 300° C.), and then a two-stage raising temperature and regeneration temperature holding process of additionally increasing the temperature to a target regeneration temperature (for example, 600° C.) are performed.
The one-stage temperature raising process may increase the temperature by opening a turbocharger bypass valve, cutting off an intake throttle valve, retarding fuel injection timing, cutting off an EGR valve, or the like, and the two-stage temperature raising process may increase a temperature of the inlet of the DPF (temperature of exhaust gas of the inlet of the DPF) to a target temperature by injecting fuel into the exhaust pipe through the injector and may maintain the temperature.
However, when the initial idle temperature is too low after the regeneration mode entry, for example, when an outdoor temperature is low during winter, it is difficult to increase the temperature and secure a regeneration temperature, and there is a problem of a reduction in a DPF regeneration success rate due to a cold condition of the overall system.
It is difficult to ensure target temperature raising performance under a low temperature condition and a warm-up time is also long, which has an adverse effect on the regeneration success rate and the regeneration period of the DPF.
Further, if a fuel injection quantity of the engine is increased to quickly increase temperature, the occurrence of white smoke is increased and if the fuel injection quantity is reduced to reduce the white smoke, it is difficult to increase temperature.
To solve the problem, an air heater installed to increase an intake temperature of the engine is used to smoothly enable the regeneration temperature at the time of the active regeneration of the DPF during the low temperature condition.
The air heater heats intake air of an intake manifold and if the air heater is installed in the intake manifold to apply a current for a predetermined time during, before or after initial starting, the air heater generates heat through heat radiation, convection or conduction, and increases the intake temperature of the engine through the transfer of the generated heat to increase the temperature of the exhaust gas while preventing the unburned combustion of the engine.
The air heater is operated at an early stage at the time of the active regeneration of the DPF during the low outdoor temperature condition to heat charge air to quickly enter the temperature condition of the exhaust gas for the regeneration.
However, when the air heater is used for a long time in warmer conditions, a temperature around a relay may be greatly increased due to heat generation of the relay turning on/off an operation of the air heater, that is, a heater relay, such that heat damage or damage to parts, such as a cover, around the relay may occur.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the disclosure and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.